101 FXUS20 KWBC 131912 PMDSA South America Forecast Discussion - International Desks NWS Weather Prediction Center College Park MD 311 PM EDT Mon Apr 13 2026 GFS data at ftpprd.ncep.noaa.gov/data/nccf/com/gfs/prod/ Forecast Bulletin 13 April 2026 at 1915 UTC: Tropical South America... In the big scale, the divergent phase of the Madden-Julian Oscillation (MJO) is expected over the region over the next work week and into next week. The divergent phase of the MJO may favor enhanced deep convection over regions with the synoptic conditions present, which could also be reflected in heavy precipitation totals. The northern sector of South America is seeing the propagation of moist plumes over the Amazon Basin, traveling through the ITCZ over the northern coast of Brasil and the Atlantic Ocean. In the upper levels, anticyclonic rotation is expected to remain over much of the western Amazon Basin, and weaken over the next three days as a shortwave upper level trough enters through south Peru and north Chile from Monday into Tuesday. In addition, an upper trough extends from the southern Atlantic Ocean into eastern Brasil, favoring general dry conditions, with the exception of a few moist plumes traveling the trade winds and entering along the Brasilian Nordeste and Bahia. With the divergent MJO over the region, expect some enhanced daily precipitation totals over portions of the western Amazon Basin, especially over the regions where the troughs are expected. Some areas are expected to exceed 50mm of daily total precipitation. Subtropical South America.. A progressive troughing pattern is expected to enter the region over the next several days, with short wave troughing anticipated over much of northern Argentina and Paraguay. On Monday, a positively tilted upper level trough enters northern Chile/south Peru by the evening hours, with its divergent side over the Chaco region and northwest Argentina. With this upper level support, the pressures in the lower levels are expected to decrease and activate the South American Low Level Jet and enhance moisture transport into the region, exceeding 50mm from Paraguay into northern Argentina. Expect moderate to heavy precipitation over northern Argentina on Monday. Into Tuesday, the upper level shortwave trough continues into northern Argentina, becoming negatively tilted over the Chaco region by the evening on Tuesday. Expect increased divergence aloft, fortified moisture convergence in the lower levels, and much troughing over much of north-central Argentina over the day on Tuesday, and then propagating into Paraguay, Mesopotamia region in Argentina and northern Uruguay over the evening into Wednesday morning. With the presence of the SALLJ, expect precipitable water levels to exceed over 65mm, if not more by the evening hours throughout the region, favoring heavy precipitation in less that 24 hours for much of the region. Some model guidance is suggesting that available moisture could exceed 75mm in some localized areas, and thus expect much heavier precipitation locally, especially over northern Argentina. By Wednesday, the upper level trough is expected over Uruguay by the evening hours, and it is expected to weaken slightly. However, this trough will favor cyclogenesis in the lower levels, especially over the Rio de La Plata region. Expect enhanced northerly and easterly moist winds to enter along the coastal regions of Buenos Aires and Uruguay, favoring moderate to heavy precipitation. Extra-tropical South America... On Monday, a wide upper level trough approaches the southern cone of South America by the evening hours, with the presence of a cold front that will favor moderate precipitation totals throughout Austral Chile. It is expected that the systems will move in a progressive manner over the next few days, and thus, the upper level trough enters the region by the evening hours of Tuesday, favoring an air mass that contains less available moisture over the region on Tuesday. Expect light precipitation totals in Austral Chile for Tuesday. By Wednesday, the region can expect to see an increasing trend of moisture with the entry of a weak atmospheric river along the Austral region of Chile. However, this increase is expected at the later part of the day, and moving quickly northward into central Chile by Thursday. As such, expect light to moderate amount in the Austral region of Chile on Wednesday. Castellanos...(WPC) $$